Present day mask, reticle or integrated circuit processing techniques often employ lithography processing, such as electron beam, laser and X-ray lithography, to write ultra small geometries onto a workpiece. The structures on the workpiece (a semi conducting substrate or chromium on a transparent substrate) are formed by exposure to electromagnetic radiation or an electron beam of a photosensitive or electron-sensitive covering on the workpiece.
A wafer may be processed either by direct writing or using masks and/or reticles. Wafers may be exposed to ultraviolet light through one or a plurality of masks and thereby transferring the pattern formed on the masks onto the wafer.
A common aspect of all known pattern generators is that the pattern is described in digital data bank containing a list of all structure or pattern elements with their geometrical data. Before the structures are written, the geometrical data are converted to a format, which is used by the writing hardware. During that conversion operation geometrical coordinates are truncated to the addressing resolution of the hardware, that is to say the writing address grid.
Most modern pattern generators use a raster scan principle with a scanning beam which is either an electron beam, X-ray beam or a laser beam and which is deflected along parallel lines on the substrate which is covered with a radiation sensitive covering. The beam is switched on and off in accordance with a bitmap for the structure, which is stored in a control system. Another option is that the beam is produced during a writing time, which is derived from a data stored in an intermediate compressed format. A pattern generator of the kind described above can be found in patent application WO 98/33096, filed by the same assignee as the present invention.
Another type of pattern generator is described in patent application WO 99/45439, also filed by the same assignee as the present application, which uses a spatial light modulator (SLM) of the micromirror type in order to produce the pattern on the workpiece. The use of an SLM in a pattern generator has a number of advantages compared to the described method above of using scanning laser spots. The SLM is a massively parallel device and the number of pixels that can be written per second is extremely high. The optical system is also simpler in the sense that the illumination of the SLM is non-critical, while in the laser scanner the entire beam path has to be built with high precision. Compared to some types of scanners, in particular electrooptic and acoustooptic ones, the micromirror SLM can be used at shorter wavelengths since it is a purely reflective device. The SLM may also be of transmissive type. The reflective SLM works in principle either in deflection mode or in phase mode, where the phase SLM extinguishes the beam in a specular direction by destructive interference, while a pixel in a deflection SLM deflects the specular beam geometrically to one side so that it misses an aperture of an imaging lens.
However, the pattern transferred to the workpiece becomes most likely different from the mask pattern, owing to diffraction of light in the corners of features in said mask pattern. Said effect is often termed as laser proximity effect.
When two areas are close together, there may be a cross dosing of the writing energy (electrons, photons), causing an undesirable increase of the adjacent portions of the written areas. This unwanted exposure of a feature by one or more of its neighbors, termed the optical proximity effect, constitutes the fundamental limit to resolution in lithography processing. Compensation or correction of proximity effect requires an alteration of the representation of an image to be patterned.
In a wafer lithography process there might be useful to correct for process dependent parameters such as that the resist is not having the same properties over the whole wafer, correction of lens artifacts and under etching etc.
In recent years, with the reduction in the size of patterns of integrated circuits, it is required to control the pattern size more precisely.
In practice, however, there occurs pattern deformation like those described above, the effect of which become increasingly important. A problem, which seems to be more and more important, is that a feature in one image will most probably be corrected differently compared to if said feature is present in another image. There is therefore a need in the art for a method, which corrects for proximity effects taking into account the pattern to be printed.